Due to its high capacity and high discharging voltage, application of lithium ion secondary batteries has been expanding as a power source for various portable devices. The battery includes a separator, which is disposed between the positive electrode and the negative electrode to electrically insulate the electrodes from each other. For the separator, microporous films comprising a polyolefin resin are mainly used.
However, the separator comprising a polyolefin resin has heat-resistance to temperature of about 120 to 160° C., and its heat-resistance is insufficient. Thus, there is a problem in that the battery temperature becomes abnormally high, when an internal short-circuit occurred in the battery or a sharply projected object such as a nail is penetrated into the battery. That is, when an internal short-circuit occurs, the separator contracts from the short circuit reaction heat to expand the short circuit portion, generating much further reaction heat.
Therefore, to solve the above problem, a method is proposed to form a porous film comprising a resin binder and a powder of an inorganic compound such as alumina, on at least one surface of the positive electrode and the negative electrode (for example, Patent Document 1). Also proposed is a method for improving the mechanical strength of a resin binder for the porous film, i.e., polyvinylidene fluoride (for example, Patent Document 2).
However, since the porous film is hard, when an electrode assembly is produced by winding the electrode having the porous film, there has been a problem in that the porous film comes off and separates from the electrode along with the material mixture layer of the electrode. Furthermore, even by simply applying the technique disclosed in Patent Document 2, the problem mentioned above is not solved.
Patent Document 1:
    Japanese Laid-Open Patent Publication No. Hei 07-220759Patent Document 2:    Japanese Laid-Open Patent Publication No. Hei 07-173323